Honestly, I think the best way is to set up every dosage problem the same way, EVERY time. It doesn't matter what the units are. Here's what I do.
Step 1: Determine what the desired result is. In this case.... gtt/min Start with that, since that's what we want to end up with.
Step 2: Look at the info provided, does any of it have either part of the desired result in it already?
Step 3: Set up the equation until all things cancel out except the desired result. Remember, what's on the bottom should cancel out with another one on the top if it is not part of the desired result.
Looking for gtt/min?
( 20gtt / 1 mL ) x ( 150 mL / hr ) x ( 1 hr / 60 min ) = left with gtt/min (mL and hr both cancel each out since one's on top the other on bottom)
(20 x 150) / 60 = 50 gtt / min
Might not work for everyone, but I was previously a chemist. So I like using the old school chem way to convert units.
NursieSD, part of your problem is that you look at these numbers and can't think how to cram them into a formula. It's a common problem.
Try this, the next time you have any sort of medication algebra (yep, that's what it is) problem.
Look at the question and break it down verbally, in your head, or on paper if you have to.
"Hmm, I have to give 150cc per hour. OK, got that."
"IV tubing makes drops. If mine makes twenty drops per cc, how many drops is that in an hour?" (huge number-- 20 x 150 = 3000)
"But wait, I don't care about the hour. How many drops in a minute? Well, there are 60 minutes in an hour. So if I divided that huge number by sixty, that would tell me how many drops in a minute." 3000 / 60 = 50.
See, it's not so hard if you know where you're going before you start running.